Michelin rejiggered the naming and it's a little confusing. The LTX series was Michelin's "truck" family, which now includes SUVs and CUVs as well. The exceptions to that rule were the Latitude/Diamaris/Agilis/Cross Terrain SUV series.
For "mainline" tires, Michelin has:
Defender - passenger/truck all-season. Defender T+H and Defender LTX. The Defender LTX is the replacement for the LTX M/S.
Premier - "premium" passenger "touring" and SUV: Premier A/S(replaces the Energy MXV4 and Primacy MXV4 series) and Premier LTX(which replaces the Latitude Tour HP)
The Michelin X series is for Costco and other warehouse clubs as an "exclusive" and to prevent a direct price comparison with other retailers. Michelin has made special tires for Sears and Discount Tire/America's Tire. The X Radial was based on whichever was their mainstream tire. The Defender XT/X Tour A/S are warehouse club versions of the Defender, X Radial LT/X LT A/S are based on the LTX M/S and Defender LTX.